The present invention relates to a method of producing an optical video disc, in particular to a method of producing a two-disc laminated optical video disc.
A conventional two-disc laminated optical video disc comprises a first disc having a first information recording surface and a second disc having a second information recording surface. The first disc and the second disc are bonded together by means of an adhesive agent of hot-melting type (a sort of hot-melt adhesive). In use, a laser beam is directed to pass through one disc (for example, the first disc) to irradiate the information recording surface thereof so as to read out the information recorded thereon. After the information recorded on said one disc (for example, the first disc) has been read out, the two-disc laminated optical video disc has to be turned upper side down so as to read out information recorded on the other disc (for example, the second disc). Alternatively, an optical head has to be moved to the other side of the two-disc laminated optical video-disc to read out information recorded on said other disc (for example, the second disc). Accordingly, an operation for reproducing a conventional two-disc laminated optical video disc is often considered to be troublesome.
In order to avoid the above troublesome operation, there has been suggested an improved tow-disc laminated optical video disc capable of reproducing information recorded on both discs by directing a laser beam in only one direction. In detail, a reflecting film formed on the information recording surface of one disc is made capable of partially reflecting and partially transmitting an incoming laser beam, whilst a middle layer of adhesive agent formed between the two discs is made complete light-transmissible.
However, in the manufacturing of the above two-disc laminated optical video disc, it is necessary either to use a liquid ultraviolet-setting resin as an adhesive agent layer, or to use a light-transmissible adhesive sheet as an adhesive agent layer.
In the former case, since it is easy for air to enter the adhesive agent layer, it is difficult to form an adhesive agent layer having a uniform thickness. In the later case, although it is easy to form an adhesive agent layer having a uniform thickness, some smuts or debris which are possible to exist between a disc substrate and a translucent reflecting film, will exert a stress force on the reflecting film, resulting in a problem such as a crack in the disc.
It is an object of the present invention to provide an improved method for efficiently producing a two-disc laminated optical video disc capable of reproducing information recorded on both discs by directing a laser beam in only one direction, so as to solve the above-mentioned problem peculiar to the above-mentioned prior art.
According to the present invention, there is provided a method of producing a two-disc laminated optical disc, comprising steps of: forming, on an information recording surface of a first light-transmissible substrate, a first reflecting film capable of partially reflecting and partially transmitting a laser beam; forming, on an information recording surface of a second light-transmissible substrate, a second reflecting film having a reflectance higher than that of the first reflecting film; applying a liquid ultraviolet-setting resin to the first reflecting film formed on the first light-transmissible substrate and/or the second reflecting film formed on the second light-transmissible substrate, in order to form a protection film(s) thereon; bonding together the above treated first light-transmissible substrate and the second light-transmissible substrate, with a light-transmissible middle layer interposed therebetween; solidifying the liquid ultraviolet-setting resin on the first light-transmissible substrate, by means of an ultraviolet light directed from below the first light-transmissible substrate.
According to one aspect of the present invention, before bonding together the first light-transmissible substrate and the second light-transmissible substrate, the liquid ultraviolet-setting resin is applied to the first reflecting film formed on the first light-transmissible substrate, followed by irradiation with an ultraviolet light. Alternatively, before bonding together the first light-transmissible substrate and the second light-transmissible substrate, the liquid ultraviolet-setting resin is applied to the second reflecting films formed on the second light-transmissible substrate, followed by irradiation with an ultraviolet light.
According to another aspect of the present invention, the light-transmissible middle layer is a light-transmissible sheet.
Before bonding together the first light-transmissible substrate and the second light-transmissible substrate, the light-transmissible sheet is laminated on the second reflecting film formed on the second light-transmissible substrate. Alternatively, before bonding together the first light-transmissible substrate and the second light-transmissible substrate, the light-transmissible sheet is laminated on the second protection film (hardened) formed over the second reflecting film of the second light-transmissible substrate.
On the other hand, it is also possible that before bonding together the first light-transmissible substrate and the second light-transmissible substrate, the light-transmissible sheet is laminated on the first reflecting film formed on the first light-transmissible substrate. Alternatively, before bonding together the first light-transmissible substrate and the second light-transmissible substrate, the light-transmissible sheet is laminated on the first protection film (hardened) formed over the second reflecting film of the first light-transmissible substrate.
According to a further aspect of the present invention, the liquid ultraviolet-setting resin has a viscosity of 15-400 cps under room temperature and in an unhardened state, and a thickness of 5-10 xcexcm in an unhardened state.
According to a still further aspect of the present invention, the light-transmissible middle layer is formed by an ultraviolet-hardening film, which has a viscosity of 3,500-400,000 P under room temperature and in an unhardened state, and a thickness of 30-60 xcexcm in an unhardened state. In practice, the light-transmissible middle layer formed by the ultraviolet-hardening film is hardened by means of an ultraviolet light directed from below the first light-transmissible substrate.
According to one more aspect of the present invention, the light-transmissible middle layer is formed by an light-transmissible adhesive sheet having a thickness of 30-60 xcexcm.
According to one more aspect of the present invention, the first reflecting film capable of partially reflecting and partially transmitting a laser beam, is a metal film or a dielectric film.
According to one more aspect of the present invention, the step of bonding together the first light-transmissible substrate and the second light-transmissible substrate, is carried out in a vacuum condition.
According to one more aspect of the present invention, said method further comprises a step of using an ultraviolet light to irradiate the unhardened ultraviolet-setting resin applied to the first reflecting film formed on the first light-transmissible substrate, except a part of the resin applied to an outer circumferential area of the reflecting film.
According to one more aspect of the present invention, the first light-transmissible substrate and the second light-transmissible substrate are bonded together first in a vacuum condition, then by means of a pressure which is so adjusted that there will be no any outward movement of the liquid ultraviolet-setting resin.
According to one more aspect of the present invention, the first light-transmissible substrate and the second light-transmissible substrate are bonded together first in a vacuum condition, then by means of press using a high pressure air.